The present invention relates to a method for obtaining urea prills.
More in particular, the invention relates to a method for obtaining urea prills by cooling urea melt droplets in a prilling tower of a urea plant. In accordance to a further aspect, the invention concerns a prilling tower for obtaining urea prills according to the above method.
A prilling tower for producing urea prills generally comprises a vertical tubular wall, which defines an inner chamber for cooling urea melt droplets. In an upper part of the chamber a urea melt distributing device is supported. The device distributes downward, all over the tower cross section, melt urea at high temperature in the form of droplets, which fall along urea falling paths inside the chamber.
The falling urea melt droplets are then solidified and cooled down by contact with air flowing upwardly in the chamber, counter currently to the urea droplets. The cooling transforms the urea melt droplets in solid urea prills.
The urea prills thus obtained are collected in a collecting bottom comprised in a lower part of the aforesaid chamber and extracted from the prilling tower by conventional means such as conveyer belts. In particular, on such bottom a rotating scraper is generally present in order to convey the prills through an opening in the bottom on the conveyer belt located underneath the bottom.
As well known, a very important parameter to produce good quality urea prills, i.e. properly cooled and solidified prills, in a modern urea plant is to set the proper residence time within the prilling tower.
The xe2x80x9cresidence timexe2x80x9d is defined as the time period required by a melt urea droplet to cover the distance from the distributing device to the collecting bottom, where the urea droplet is collected in the form of a solid urea prill.
The bigger is said distance, the longer will be the residence time during which the cooling occurs and consequently the cooler and thus the harder and more mechanically resistant will be the urea prills falling on the collecting bottom, given a predetermined air flowrate and air temperature.
The tower wall, made by concrete or other building materials, has a relevant height to provide the aforementioned desired distance.
In this respect it should be noted that among the equipment and units of a plant for the production of urea such as reaction, decomposition and condensation units the prilling tower represents the most extensive one.
This is because of the aforesaid relevant height required to obtain the necessary residence time for the urea melt droplets at high temperature to be cooled down.
However, in the field of urea prilling towers the need is increasingly felt of keeping their height as low as possible to reduce construction complexity, investment and maintenance costs as well as to increase the tower reliability. This means that the residence time should be reduced as much as possible without negatively effecting the cooling degree in order to guarantee satisfactory solid urea prills.
To this aim, few options are available according to the prior art as will be apparent in the forthcoming description.
Such options are also generally applied in case the production capacity of a pre-existing urea plant has to be increased and thus a greater amount of urea heat has to be removed from the prilling tower in order to maintain an appropriate cooling of the urea melt droplets.
A first option is to increase the air flowrate within the tower to enhance the droplets cooling. There is, however, a limitation to such increase because a too high flowrate could lift the urea droplets upwards causing a carry over of such droplets in the chamber. In addition, this option is often technically nor possible or very difficult to be carried out.
A second option is to chill the air flowing through the tower prior to entering the tower.
The cooling equipment required to cool the air flow is however a refrigerator system working on large air-volumes with a high-energy consumption that requires high investments and maintenance costs in addition to the tower costs.
A third option is to cool the urea prills at the tower bottom or outside the tower by means of a fluid bed cooler.
To obtain this cooling it is required a fluidising air flow, for instance at the tower bottom, having a pressure high enough to fluidise the prills suspended in a fluid bed and with a temperature low enough to cool the prills.
This involves an air booster compressor and an appropriate refrigerator system for said fluidising air flow, with ensuing high investment costs and energy consumption.
To overcome the above drawbacks a fourth option proposes the use of a cooling device outside the tower in fluid communication with the collecting bottom and downstream thereof.
Said cooling device, made by parallel disposed cooling plates internally fed with a cooling fluid, is crossed by the urea prills that are further cooled down by the contact with the external cold surface.
This device requires less energy and less capital investments then the cooling equipment previously described.
However, some additional equipment are still required to bring the prills from the tower collecting bottom to the cooler, to lift the prills to the cooler entrance and to screen the prills, which render the cooling device technically complex to manufacture and costly.
It follows that in the prior art no effective solution has been proposed in the aforesaid field of application, to enhance the urea prills cooling efficiency in a very simple, reliable and cost effective way, notwithstanding the even increasingly felt need of minimizing the height of new prilling towers or to increase the cooling efficiency of existing towers after a production increase while reducing or maintaining unaltered the tower height.
The technical problem underlying the present invention is to provide a method for obtaining urea prills in a prilling tower with improved cooling efficiency of the urea melt droplets which is simple to carry out, reliable and does not require high investment and operating costs.
The problem is solved according to the present invention by a method for obtaining urea prills in a prilling tower, comprising the step of:
making a plurality of melt urea droplets to fall from a urea melt distributing device towards an urea prills collecting bottom of the prilling tower;
characterized by the fact of further comprising the step of:
cooling said collecting bottom.
According to a further aspect thereof, the present invention makes available a prilling tower for obtaining urea prills comprising a melt urea distributing device and an urea prills collecting bottom characterised by the fact of further comprising:
means for cooling the collecting bottom.
Thanks to the present invention, the cooling of the urea prills is advantageously substantially increased in an easy and reliable way.
In fact, cooling of the urea prills falling on the collecting bottom can be effectively and satisfactorily completed directly on the latter. This advantageously allows to manufacture prilling towers of reduced height, i.e. with a residence time of the falling urea melt droplets far from the optimal one, and to increase the production capacity of an existing tower without the need of increasing its height.
Moreover, the present invention can be easily applied to both new or existing prilling towers without requiring particular structural changes in the tower itself as well as high investment and operating costs.
The features and advantages of the invention will be clear from the following indicative and non-limiting description of an embodiment of the invention, made with reference to the attached drawings.